One known tunnel boring machine for use in excavation of a rock-bed is designed to have a cylindrical cutterhead at the front part of the machine body and this cutterhead includes a circular cutting face at its front face with respect to an advancing direction. Such a tunnel boring machine includes a pair of gripper assemblies that are slidable back and forth relative to the tunneling machine body and are interconnected to the tunneling machine body by means of thrust cylinders. As the cutterhead rotates, the thrust cylinders extend with the gripper assemblies being pushed against the wall of the tunnel, which allows the tunneling machine body to advance while excavating. After the advance of the tunneling machine body with a specified pitch for excavation, the gripper assemblies are released from its pressed condition and then moved forward relative to the tunneling machine body by the contraction of the thrust cylinders. With this procedure, continuous advance of the tunnel boring machine is carried out. Such a tunnel boring machine is usually used for excavating tunnels having a circular cross-section such as water channels, aqueducts and sewers.
The above tunnel boring machine is however unsuitable for use in excavating tunnels for double track railways and roads which require a space of a rectangular cross-section, and if the above tunnel boring machine is used for excavating such tunnels, a significant amount of superfluous excavation is incurred so that there arises a need for back filling etc. after excavation. This leads to additional cost and labor and therefore the above tunnel boring machine presents the problems of inefficiency and poor cost performance.
As an attempt to reduce the amount of superfluous excavation, Japanese Patent Laid-Open Publication No. 4-353192 (1992) proposes a tunnel boring machine for excavating a tunnel of an oval cross-section. The tunnel boring machine disclosed in this publication is designed such that a cutterhead having a circular cutting face is inclined by the telescopic motion of a cylinder member at a desired angle with respect to the tunneling machine body thereby to excavate a tunnel of a desired oval shape.
Japanese Patent Laid-Open Publication No. 7-26890 (1995) discloses another tunneling machine that has a cutterhead capable of inclining at a desired angle relative to the tunneling machine body. This tunneling machine has a pusher that exerts a pressing force on the cutterhead, and this pressing force resists a reaction force against excavation (hereinafter referred to as "excavating reaction force") acting in the direction of the periphery of the tunnel because of the inclination of the cutterhead and causes at the same time a reaction force of the tunnel wall.
The above described types of boring machines for tunnels of an oval cross-section use a cutterhead that is adjustable to incline at desired angles and therefore require a mechanism such as a cylinder member for changing the angle of the cutterhead. Since the whole of an excavating reaction force is imposed on the mechanism such as a cylinder member during excavation, the cylinder member is required to be pretty large in size. In addition, in order to maintain the angle of inclination of the cutterhead constant during excavation, a large sized and complex control mechanism is needed for controlling the cylinder member.
In the above tunnel boring machines for tunnels of an oval cross-section, the roof support (support member) which receives an excavating reaction force acting in the direction of the periphery of the tunnel because of the inclination of the cutterhead is designed to move in an integral fashion with the tunneling machine body so that an extremely great friction force is generated between the roof support and the tunnel wall during the advance of the tunneling machine body. This friction force works as a resistance to the tunneling machine body during its advance, which hinders the smooth advance of the tunneling machine body.
The present invention has been made for the purpose of overcoming the above problems and a prime object of the invention is therefore to provide a tunnel boring machine for use in excavation of a tunnel of an oval cross-section, which is simple in structure.
Another object of the invention is to provide a tunnel boring machine for use in excavation of a tunnel of an oval cross-section, which is capable of reducing a friction force generated between the tunnel wall and a support member that receives an excavating reaction force working in the direction of the periphery of the tunnel.